1. Field of the Invention
The invention relates to a rare earth magnet constituted by a polycrystal having nanosize crystal grain size and to a manufacturing method therefor.
2. Description of the Related Art
Rare earth magnets such as a neodymium magnet (Nd2Fe14B) have been used for various applications as very powerful permanent magnets with a high magnetic flux density. A nanosize crystal grain size should be ensured to obtain even better magnetic properties.
A technique by which a melt of a magnet composition is rapidly cooled by a single roll method or a twin roll method to obtain a thin strip has been used to realize a nanosize crystal grain size. Where the cooling rate during rapid cooling is too high, an amorphous structure is obtained in the entire magnet or part thereof. The amorphous structure can be crystallized by appropriate annealing, but the grain size in this case becomes larger than that of the crystal structure directly formed by rapid cooling.
In sintered magnets using multi-domain Nd2Fe14B particles with a size of about several micron, a grain boundary phase should be present as a barrier preventing the movement or appearance of magnetic walls in order to realize a high coercive force.
For example, Japanese Patent Application Publication No. 2007-251037 (JP-A-2007-251037) and Japanese Patent Application Publication No. 2008-069444 (JP-A-2008-069444) disclose a method including the steps of feeding an alloy melt containing praseodymium (Pr), neodymium (Nd), iron (Fe), cobalt (Co), niobium (Nb), yttrium (Y), and boron (B) to a rotating cooling roll, rapidly cooling, obtaining a thin strip, and crystallizing the thin strip by heat treating at a temperature rise rate of 150 to 250° C./min. As a result, a thin alloy strip for a rare earth magnet constituted by a polycrystal and including the aforementioned constituent element can be obtained.
However, no consideration is given to a graph boundary phase and there is room for increasing a coercive force.
A coercive force of the aforementioned magnet at room temperature has been evaluated, but for a motor of a hybrid vehicle the evaluation should be conducted for a coercive force at a temperature close to 160° C. which is in the usage temperature range of the motor.